We’ll probably never know the exact reasons, but it is pretty clear that Rudolf was not a very happy man. Here are some of the factors that probably played a role in his eventual suicide:
Rudolf did not get along with his father, Franz Josef. His personality and temperament was very different from Franz Josef’s. Franz Josef wanted an athletic and active son who would grow up to be a good soldier and leader and Rudolf was sensitive and intellectual. Furthermore, Franz Josef and Rudolf did not agree about politics. Franz Josef was a reactionary who favored absolute monarchy, while Rudolf was a liberal inclined to favor constitutional monarchy or democracy. Rudolf was also sympathetic to the Hungarians while Franz Josef was not.
Rudolf did not have a happy childhood. Like most Austrian princes, he was given a military education that emphasized physical fitness and discipline. While that wasn’t unusual for the time, Rudolf’s tutor, Leopold Gondrecourt, seems to have been far stricter than usual, because Rudolf’s father and paternal grandmother wanted to toughen him up and make him fit for the role that he was born to play. Empress Elisabeth eventually managed to convince Franz Josef to dismiss Gondrecourt, mostly because Elisabeth and several other members of the court were convinced (not without reason) that Gondrecourt would kill Rudolf with his strictness.
Rudolf had a distant relationship with his mother, Elisabeth. He was fond of his mother (and very similar to her in personality, temperament and political views), but they weren’t especially close. Elisabeth was not initially allowed to play much of a role in the lives of Rudolf or his elder sisters, Sophie and Gisela, by her mother-in-law, Archduchess Sophie. Elisabeth frequently left the Austrian court and traveled abroad because she couldn’t take the strictness of court protocol. Elisabeth spent so much time abroad that Rudolf and Gisela were essentially strangers to her by the time they reached adolescence (Sophie died young). Furthermore, Elisabeth’s favorite child was her youngest daughter, Valerie, and she made little secret of it.
Rudolf’s marriage to Princess Stéphanie of Belgium was a troubled and unhappy one. They got along well at first, but things when downhill after the birth of their only child, Archduchess Elisabeth, when it became clear that Rudolf and Stéphanie were too different to be compatible. Stéphanie had also had an unhappy childhood and didn’t get along with her father, Leopold II of Belgium, but she was far more stable, formal, and conventional than Rudolf and politically, she was conservative. It didn’t help that Rudolf may have also infected Stéphanie with a sexually transmitted disease that left her infertile.
Rudolf most likely suffered from clinical depression. This is one of the biggest factors, in my opinion, and I think it underpins many of the other factors. Empress Elisabeth probably also suffered from clinical depression and she pretty clearly had an eating disorder as well. Furthermore, Elisabeth’s cousins were Ludwig II and Otto of Bavaria; Ludwig was very eccentric (though there’s no consensus about the state of his mental health) and Otto suffered from what sounds like depression and panic attacks.
Rudolf himself had a nervous and melancholy disposition and like many people with depression, he was also an alcoholic who probably turned to drinking to numb the symptoms of depression. While drinking can provide temporary relief, in the long term, it makes depression worse, because alcohol is a depressant.
Sources:
• Richard Barkeley, The Road to Mayerling: Life and Death of Crown Prince Rudolph of Austria.
• Brigitte Hamann, The Reluctant Empress: A Biography of Empress Elisabeth of Austria.
• Alan Warwick Palmer, Twilight of the Habsburgs: The Life and Times of Emperor Francis Joseph.